This invention relates to liquid, low-viscosity, homogeneous prepolymers based on toluene diisocyanate (TDI) having an NCO content of from about 2 to 16%, a viscosity of about 10,000 mPa.s or less at room temperature and containing about 1.0% or less of free monomeric TDI. The present invention also relates to a process for the preparation of these liquid, low-viscosity, homogeneous prepolymers.
Various types of isocyanate prepolymers are known and disclosed in, for example, U.S. Pat. Nos. 3,115,481, 3,600,358, 3,766,148, 3,789,032, 3,963,681, 3,997,514 and 4,098,733. Isocyanate prepolymers are commonly used to produce polyurethane and/or polyurea based products such as, for example, elastomers, foams, coatings, adhesives, caulkings, sealants, binders, etc.
TDI based prepolymers are particularly desirable as these exhibit lower viscosities at any given NCO in comparison to other commercially available aromatic diisocyanates. Some of the disadvantages commonly associated with the known TDI based isocyanate prepolymers are potential industrial hygiene issues related to the high vapor pressure caused by large quantities of free monomeric TDI. Various methods to reduce the free monomeric diisocyanate contents of isocyanate prepolymers are known and described in, for example, U.S. Pat. Nos. 3,183,112, 3,248,372, 3,384,624, 3,883,577, 4,061,662, 4,683,279.
Polyurethane casting compositions are described in U.S. Pat. No. 3,211,701. These comprise the reaction product of an isocyanate-terminated interpolymer with an organic crosslinking agent (which may be a polyamine or a polyhydric alcohol). First, a hydroxyl-terminated polyester urethane is formed by reacting a polyester having a molecular weight of 900 to 1200 with an organic diisocyanate, including mixtures of the 2,4- and 2,6-isomers of toluene diisocyanate. These hydroxyl-terminated polyester urethanes exhibit softening points of from about 38 to about 40.degree. C. The hydroxyl-terminated polyester urethane is then reacted with an excess of an organic diisocyanate (preferably naphthylene-1,5-diisocyanate, p-phenylene diisocyanate of diphenylmethane-4,4-diisocyanate) to form the isocyanate-terminated interpolymer. These isocyanate-terminated interpolymers are solids at room temperatures.
Isocyanate-terminated prepolymers are broadly described in U.S. Pat. No. 3,963,681. These prepolymers may be cured with an aromatic or aliphatic polyamine or polyol. Suitable isocyanates for the prepolymers include aromatic or aliphatic diisocyanates and triisocyanates. The polyurethane/urea elastomers of the '681 patent are based on mixtures of polytetramethylene glycol ethers (Polymegs) of two different molecular weights (one high and one low, with ca. 1,000 to 4,500 average MW of the blend). These blends of polyols are described as resulting in better high temperature and dynamic properties than a single Polymeg having the same average MW as the above blend.
Prepolymers and polyurethane/ureas produced therefrom are described in U.S. Pat. Nos. 3,766,148 and 3,600,358. The prepolymers in both patents are based on methylene bis(4-phenylisocyanate). The '148 patent describes chain extending these methylene bis(4-phenylisocyanate) based prepolymers with MDA (4,4'-methylenedianiline) to form poly- urethane/ureas. U.S. Pat. No. 3,600,358 describes prepolymers based on methylene bis(4-phenylisocyanate) with neopentyl adipate (or other esters of neopentyl glycol). These prepolymers are subsequently chain extended with aromatic diamines, preferably MDA, to form polyurethane/-urea elastomers.
U.S. Pat. No. 3,115,481 also describes polyurethane/urea elastomers. These elastomers are formed by first preparing foams and subsequently crushing the cells in a heated press. This invention uses prepolymers wherein an aromatic isocyanate is present on the prepolymer ends. This increases the reactivity of the isocyanate prepolymers such that they are suitable for reaction with water to form foams. However, this high reactivity makes this type of prepolymer too fast to process with diamine chain extenders to form cast elastomers.
Polyurethane/urea elastomers are also described in U.S. Pat. No. 3,789,032. These elastomers are aromatic isocyanate terminated prepolymers like those of the '481 patent. The elastomers produced in this reference are also produced by reacting the aromatic isocyanate terminated prepolymers with water. Increased reactivity is also a problem in these prepolymers.
Isocyanate prepolymers having good storage stability and based on a mixture of polyisocyanates with a reactive hydrogen containing material are disclosed in U.S. Pat. No. 3,637,599. Suitable polyisocyanates include polyphenyl polymethylene polyisocyanate and toluene diisocyanates, and suitable reactive hydrogen containing materials have molecular weights of from 500 to 6000. The working examples only use polyesters as the reactive hydrogen containing material for preparing the isocyanate prepolymers. The produced prepolymers have a high free TDI monomer content and are solids at room temperature.
U.S. Pat. No. 4,098,773 describes prepolymers that are prepared from linear polyols and aliphatic isocyanates, cycloaliphatic isocyanates, aliphatic-aromatic isocyanates, sterically hindered aromatic isocyanates or 4,4'-methylene bis(phenylisocyanate) where the OH:NCO ratio is 1:1.1 to 1:2. A symmetrical aromatic diisocyanate is subsequently added to the prepolymer, and the prepolymers are chain extended with symmetric diols to prepare polyurethanes with high softening points. The working examples of the '773 patent use 4,4'-diphenylmethane diisocyanate, isomeric mixtures of toluene diisocyanate and hexamethylene diisocyanate, and typically a polyester as the linear polyol. Only Example 8 uses a polyether, more specifically Polymeg 2000. The prepolymer of this example exhibits the following properties: an NCO content of 7.0% and a viscosity of 14,370 mPa.s at 25.degree. C. The viscosity of this particular prepolymer clearly exceeds that required by those of the present invention.
U.S. Pat. No. 3,997,514 describes isocyanate terminated prepolymers prepared from mixtures of aromatic and aliphatic diisocyanates. These prepolymers comprise the reaction product of excess polyesters or polyether polyols with an aromatic diisocyanate to form a hydroxyl terminated prepolymer. This OH terminated prepolymer subsequently reacts with an excess of aliphatic diisocyanate to form a diisocyanate terminated prepolymer. These prepolymers are always terminated with an aliphatic or alicyclic diisocyanate group. In no case can prepolymers be obtained which have terminal aromatic isocyanate groups as it is the case in the present invention. These diisocyanate terminated prepolymers have high viscosities at temperatures as high as 100.degree. C., and at room temperature, either have very high viscosities or are usually solids. These prepolymers of U.S. Pat. No. 3,997,514 can be prepared in either a one-step or two-step process.
Among the various processes which have been developed in attempts to reduce the quantity of unreacted monomeric isocyanate contents in various isocyanate prepolymers are processes and/or methods using falling film evaporators, wiped film evaporators, distillation techniques, various solvents, molecular sieves, etc. Most of these processes and/or methods require an additional processing step in which the unreacted diisocyanate is removed. For example, U.S. Pat. No. 4,061,662 removes unreacted TDI from an isocyanate by contacting the isocyanate with molecular sieves. Additional processing steps require additional time to yield the desired prepolymer and increase the cost of the prepolymer.
Processes related to removing excess isocyanate from isocyanate prepolymers by a solvent and/or extraction technique include those described in, for example, U.S. Pat. Nos. 3,248,372, 3,384,624 and 3,883,577. The '372 patent discloses heating the isocyanate prepolymer under alkaline conditions to make a polymeric organic polyisocyanate soluble in organic solvent and containing less than about 1% of monomer. The excess diisocyanate is then separated by subjecting the mixture to a vacuum treatment or extraction with a solvent. Benzyl alcohol is disclosed as a suitable component to remove excess monomeric TDI in U.S. Pat. No. 3,384,624. The '577 patent discloses acetonitrile as a suitable solvent medium to remove the volatile diisocyanate.
It is also possible to distill an isocyanate prepolymer to remove the unreacted diisocyanate according to U.S. Pat. No. 4,385,171. It is necessary to use a compound which is only partially miscible with the prepolymer and has a higher boiling point that of the diisocyanate to remove the unreacted diisocyanate.
U.S. Pat. Nos. 3,183,112, 4,683,279, 5,051,152 and 5,202,001 describe falling film and/or wiped film evaporation. In U.S. Pat. No. 3,183,112, the unreacted diisocyanate is removed from an isocyanate prepolymer by allowing the prepolymer reaction mixture to flow as a thin film and heating to about 150.degree. C., while agitating the film. Advantageously, a solvent such as o-dichlorobenzene may be pumped into the bottom of the falling film evaporator to aid in removal of the unreacted diisocyanate.
Urethane-linked mixtures of 2,4- and 2,6-TDI mixtures which exhibit low melting points are disclosed by U.S. Pat. No. 4,683,279. TDI is reacted with low molecular weight polyols having from 4 to 10 carbon atoms. The excess diisocyanate may then be removed by distillation using a wiped film evaporator (see Example 1).
U.S. Pat. Nos. 5,051,152 and 5,202,001 disclose improved processes for reducing the amount of residual toluene diisocyanate in a polyurethane prepolymer reaction product mixture. The prepolymer is prepared by reacting an organic polyisocyanate with a polyol by conventional procedures. It is preferred that the NCO:OH equivalent ratio be in excess of about 2:1. This process comprises passing the prepolymer reaction product mixture through a wiped film evaporator, while adding an inert gas, specifically nitrogen, to the distillation process to sweep out the polyisocyanate to yield a prepolymer product containing less than about 0.1% by wt. of residual diisocyanate, preferably toluene diisocyanate (TDI). The inert gas, preferably nitrogen, is passed in a countercurrent flow through the evaporation zone at a specified ratio of mass flow rate of inert gas to mass flow rate of prepolymer.
U.S. Pat. No. 5,646,230 is directed to isocyanate-terminated prepolymers having a viscosity of less than about 6000 mPa.s, preferably less than about 3500 mPa.s at 80.degree. C. and an NCO content of about 3 to 10%. These prepolymers comprise the reaction product of a) an aromatic diisocyanate, b) a polyether polyol having an OH number of about 25 to 125 and containing from about 1.8 to 2.5 hydroxyl groups, and c) an aliphatic diisocyanate. In the preparation of the prepolymer, the equivalents ratio of the polyether polyol to aromatic diisocyanate is from about 1.0:0.7 to 1.0:1.1, and the total ratio of equivalents of isocyanate to polyol in the prepolymer is from about 2:1 to 4:1. The preferred aromatic diisocyanate is 4,4'-diphenylmethane diisocyanate, and the preferred aliphatic diisocyanate is bis-(4-isocyanatocyclohexyl)methane. All of the working examples use this combination of diisocyanates in the prepolymers of this application. The prepolymers made by this process will contain aliphatic or alicyclic NCO groups. However, the process of U.S. Pat. No. 5,646,230 does not allow the preparation of aromatic NCO-terminated prepolymers, which are a result of the presently claimed invention.
Advantages of the low-viscosity, low free monomer TDI containing prepolymers of the present invention include the fact that highly flexible elastomers and binders can be produced from these. Due to the possibility to produce prepolymers with free monomer content even lower than 0.3%, these products can even be spray applied to form a seamless flexible composite mat, i.e. to provide a secondary containment, or seamless liners for irrigation ditches or ponds. The prepolymers of the present invention can not be obtained by the standard one-step process which involves charging a polyol (or a mixture of polyols) to a reactor containing a mixtures of TDI and MDI (and/or polymer MDI) with stirring and heating until the reaction is complete. Prepolymers made by the standard one-step process will always exhibit unacceptably high monomeric TDI contents.
TDI based prepolymers are desirable because they result in the lowest viscosity at any given NCO-content when compared to other commercially available aromatic diisocyanates. However, these prepolymers also contain high concentrations of free TDI monomer that are objectionable from an industrial hygiene standpoint. Reducing the concentration of free TDI monomer by a thin film evaporation process is both costly, and results in a higher viscosity product.
Diphenylmethane diisocyanate (MDI) or polymeric based prepolymers prepared from polyethers are not acceptable because of the high viscosity or even solid nature of the prepolymers and the lack of high flexibility in elastomers produced from polymeric MDI prepolymers.
Prepolymers according to the present invention having terminal NCO groups derived from both TDI and MDI have the additional advantage of providing longer potlife when compared with pure MDI prepolymers. This is a definite advantage when these prepolymers are used as moisture curing binders, i.e. to bond rubber crumbs in the production of rubber tiles and synthetic sporting tracks.
Prepolymers prepared by reacting 1) a polyol with 2) a mixture of TDI and MDI (4,4'-MDI) are also unacceptable. Due to the high reactivity of MDI vs. TDI, this process forms a prepolymer which contains the MDI/polyether adducts and a high concentration of free TDI monomer. Furthermore, these prepolymers are usually very high in viscosity.